This invention relates generally to instrumentation used in the testing and measuring of electrical signals. More specifically, it relates to a high-impedance passive voltage probe for making the electrical connection between a signal under test and the input of an oscilloscope, logic analyzer, spectrum analyzer, or other type of wideband test equipment.
The ideal voltage probe would have an unlimited bandwidth and would not alter the signal being tested. It would be perfectly accurate, completely reliable, easy to use, and inexpensive. Needless to say, such an ideal voltage probe does not exist. The design of a practical voltage probe requires that compromises be made among the various probe attributes according to their relative importance. For a given measurement situation, a user must select a probe according to the measurement objective, the characteristics of the circuit under test, and the characteristics of the probe.
Wide bandwidth voltage probes can be categorized into three general types; namely: low-impedance passive probes, active probes, and high-impedance passive probes. Each of these types has a different set of inherent advantages and disadvantages.
The simplest type of probe for high-bandwidth signals is the low-impedance, or low-Z, probe. This type of probe employs a 50 ohm coaxial cable that is connected to a 50 ohm input of the test instrument. A series resistor is often used in the probe tip in order to minimize the effect of the probe on the test signal. This causes an attenuation of the test signal by the voltage divider that is formed by the tip resistor and the 50 ohm input of the test instrument. For example, a 10X probe (i.e., a probe with an attenuation factor of 10:1) would utilize a resistor of 450 ohms in the probe tip.
The low-impedance passive probe type is rugged, reliable, and relatively inexpensive. It is also capable of very high bandwidth (well into the GHz range). The only drawback of this probe type is the significant load that it introduces to the circuit under test. The load for a 10X probe is equivalent to a 500 ohm resistor to ground. This load can significantly alter the test signal, perhaps enough to cause a malfunction in the circuit being tested. consequently, the low-impedance probe type is normally used only when a very high bandwidth is required and only for circuits that can tolerate the significant resistive load of these probes.
By incorporating an amplifier circuit within the probe tip, the active probo type provides a much higher input impedance than that of the low-impedance probe type. This impedance comprises a relatively high resistance that is in parallel with a small capacitance. Because of this capacitance, which may be on the order of 1 pF, the load impedance of the active probe is frequency dependent--being lower for the higher frequency components of a test signal. Although the bandwidth capability of the active probe type is not as high as for the low-impedance probe type, it can extend well into the GHz frequency range.
A major disadvantage of the active probe type is that it is very expensive relative to either the low-impedance or high-impedance passive types. It is also less reliable and less rugged than these other types, and it requires that power be supplied to the probe. It can also introduce a DC offset error in the measured signal.
The most commonly used type of wide bandwidth voltage probe is the high-impedance passive probe. This probe type is rugged, reliable, simple to use, and inexpensive. For relatively low-frequency signals, including DC, these probes can present a very high-impedance load to the circuit under test. However, they have a significantly higher capacitive loading than the active probe type, and are not truly "high impedance" for the higher frequency components of a test signal. Therefore, connecting this probe to a test signal can attenuate the higher frequency components of the signal, thereby distorting the signal under test. For example, a signal comprising a pulse with very short-duration rise and fall times would have these times increased by the connection of the voltage probe. The relatively high input capacitance of this probe type can also impede the measurement of fast pulses by introducing spurious oscillations in the signal presented to the input of the instrument. This can occur because the input capacitance at the probe tip forms a resonant circuit with any parasitic inductance in the probe ground lead. A lower input capacitance, as is generally the case for active probes, minimizes this effect by increasing the resonant frequency, thereby making reliable measurements less sensitive to the length of the probe ground connection. Another significant disadvantage of the high-impedance probe type is that the maximum bandwidth for these probes is lower than that of the active probe type. For example, the bandwidth for a 10X high-impedance passive probe that has an acceptably low level of signal aberrations has heretofore been limited to about 500 MHz.
The disadvantages of the high-impedance passive probe type are related to the relatively low input impedance of this probe for the higher frequency components of a test signal. A high-impedance passive probe without this disadvantage would be highly desirable. Such a probe would be more competitive with active probes in regard to bandwidth and input impedance while maintaining all of the advantages associated with passive probes; such as: low cost, high reliability, and ease of use. Such a probe requires a different design approach than has been used in the prior art.
Oscilloscopes and other test instruments that are designed for use with high-impedance passive probes incorporate high-impedance inputs for making connections to these probes. The standardized high-impedance input consists of a 1 megohm resistance to ground with a parallel capacitance. The parallel input capacitance generally falls within the range of about 6 to 35 pF, with higher bandwidth instruments having an input capacitance near the lower end of this range. High-bandwidth probes incorporate a series resistance in the probe tip that attenuates the signal under test by means of the resistive voltage divider that is formed with the 1 megohm input resistance in the instrument. A 10X probe, which attenuates the signal by a factor of 10:1, requires a tip resistance of 9 megohms.
In order to achieve a high bandwidth, the tip resistance must be bypassed by a parallel tip capacitance. Otherwise, higher frequency components of a test signal would be greatly attenuated by the total capacitance at the input of the instrument. This total capacitance to ground at the instrument includes not only the input. capacitance within the test instrument, but also the distributed capacitance between the signal line and the ground return line in the probe cable. Some probes also have discrete capacitance between the signal and ground return lines, and this is also included in the total input capacitance. The total input capacitance, in concert with the tip bypass capacitance, comprises a capacitive voltage divider for the AC components of a test signal. This voltage divider must have the same attenuation factor for AC as does the resistive divider for DC. For a 10X probe, this requires that the value of the tip bypass capacitance be equal to one ninth of the value of the total input capacitance at the instrument. For some voltage probes, the tip bypass capacitance employs a variable, capacitor so that the user can adjust the frequency compensation by matching the attenuation of the capacitive voltage divider to that of the resistive voltage divider. Other probes may include a variable capacitor at the instrument end of the probe assembly in order to provide for this adjustment.
Even with the inclusion of the tip bypass capacitor, the probe just described would only be suitable for higher bandwidths if the length of the probe cable is kept very short. Otherwise, the cable behaves as a transmission line, rather than as a lumped capacitance. The problem that results from this can perhaps be best understood by considering the response of the probe to a step function.
The initial response of the probe to a step input is governed by the tip capacitance, the transmission line cable, and the capacitance at the instrument input. The tip resistor and the resistance to ground within the instrument can be ignored. A step input introduces a very narrow voltage spike at the tip end of the transmission line cable. The rise time of this spike is the same as that of the stop input--with zero rise time for an ideal step function from a source having zero impedance. The fall time is an exponential decay having a time constant given by the value of the tip capacitance multiplied by the sum of the characteristic impedance of the transmission line plus any source resistance.
For a 1 volt step input, a 10X probe must be charged to a value of 0.1 volts. However, the initial voltage spike travels down the transmission line without charging the line itself. Instead, the charge associated with this voltage spike is delivered to the instrument end of the probe. This overcharges the input capacitance at the instrument to a much higher voltage than the desired 0.1 volt step. Since the cable is not terminated in its characteristic impedance, a reflection occurs from this incident signal. Also, the overcharged input capacitor subsequently discharges back into the transmission line, which eventually lowers the input voltage to a value less than the desired 0.1 volts. A series of reflections occurs from both ends of the transmission line that results in a damped oscillation at the instrument input that eventually settles to the correct DC voltage level of 0.1 volts. Thus the input signal to the instrument does not accurately replicate the input stop function.
The most common solution to this transmission line problem in the prior art is to use a resistive conductor for the signal line of the transmission line cable (see U.S. Pat. No. 2,883,619, Kobbe, et al.). one effect of such a lossy transmission line is to attenuate the initial voltage spike as it travels down the line so that it does not appreciably overcharge the input capacitance. After the initial charge, the input will still exhibit damped oscillations about the final value. However, the amplitude and the time duration of these signal aberrations can be limited to an acceptable level.
Another solution to the problem is to add a resistor in series with the bypass capacitor at the probe tip (see U.S. Pat. Nos. 4,978,907, Smith, and 5,172,051, Zamborelli). This can limit the amplitude extend and the time duration of the initial voltage spike. Although this approach can limit the signal aberrations at the instrument input, it also increases the rise time of the initial step at the instrument input, thereby reducing probe bandwidth.